Hoist systems, in the prior art, are used in the offshore industry in the form of drilling derricks on, for example, drilling vessels. When, in use, a drill string is attached to the bottom side of a trolley, also known as a traveling block. The trolley runs on a separate track inside the derrick. These tracks must be supported in order to avoid unwanted movements of the running trolley; however, due to constructional limitations, a certain movement remains. Since the trolley is usually located inside the derrick, access to the running trolley is severely limited. This decreases the useful work that can be done with the derrick.
It is therefore advantageous for running tracks to be located on the outside of the load carrying construction. This outside location would increase the useful work that can be done. The running tracks can be integrated into the load carrying structure to obtain a satisfactory level of stiffness without adding to much extra weight to the construction.
It is therefore advantageous to use a tube or sleeve type construction as hoist system and build the running tracks on one side of the tube or sleeve. This type of construction would not have the afore-mentioned draw backs. The current application has a hoist system in the form of a tube or sleeve.
According to the prior art, it is customary for a hoisting cable to be attached to a fixed point at one end. The other end of the hoisting cable is then wound around a winch. If this winch breaks down, it is no longer possible to work with the device. The design of the afore-mentioned winch must be relatively large and costly to meet with all the required demands.
A major factor in the wearing of the hoist cable is that repeated bending of the wire in the same places. In order to increase the service life of the cable, the cable has to be shifted often. Hoist systems in the prior art use a procedure known as the “slip & cut”. This procedure requires considerable time and is also dangerous to the workers.
An object of this invention to provide a hoist system in which an increased level of redundancy is provided. Another object of this invention to provide means with which the time consuming and dangerous “slip & cut” procedure can be avoided altogether. Another object of this invention is to provide a hoist system with relatively inexpensive winches in order to decrease the building and operating cost of the hoist system.
An advantage of the invention is that hoisting means can be provided with two winches, each end of the hoisting cable being wound onto a separate winch. By winding the two ends onto a separate winch, it is possible to achieve the same cable speed at a relatively low speed of revolution of the winches. By using two winches the cable can be shifted automatically a distance from one winch to the other winch. This method effectively replaces the “slip & cut” procedure. This alternative method takes considerably less time and can be performed completely automatic reducing the chance of personal injuries.
Moreover, by adding the second winch, redundancy is provided in the system. If one of the winches fails, the hoist system is still usable and work can continue with a single winch.
The winches can be driven by a plurality of relatively small motors; therefore, twice as many sides of the winches can be used to attach the motor on the winch. These winch motors can be relatively small. For example, it is possible to equip the winches on both sides with electric motors that engage with a pinion in a toothed wheel of the winch. The first advantage of this is that such electric motors are commercially available. For the use in the hoist system, it is not necessary to develop a special and expensive hoisting winch. The second advantage is that the relatively small motors have a low internal inertia. The low inertia means that when the direction of rotation of the winch is reversed less energy and time are lost during the reversal.
In the case of a hoist system according to the prior art of the type mentioned in the preamble, finding the optimum compromise between speed and power is a known problem. The hoisting cable is guided in such a way over the cable blocks in the mast and on the trolley that several cable parts extend between the mast and the trolley. In this case the more wire parts are present between the mast and the trolley; the greater the load that can be lifted with the hoist system if the hoisting winch remains unchanged. However, the more wire parts are present between the mast and the trolley, the lower the speed at which the trolley can be moved relative to the mast when the maximal speed of the winch stays the same.
In order to find a good compromise between speed and lifting power, it is generally decided to provide the hoist system with relatively heavy winches. The heavy winches are designed to ensure that the rapid movement of the trolley up and down can be met in every case. This also means, however, that a substantial part of the lifting power is not being utilized for a substantial part of the time. In other words, the device is actually provided with winches that are too heavy—and therefore too expensive—to be able to reach sufficient speed occasionally.
Another object of the present invention to provide a hoist system of the type mentioned in the preamble. On the one hand, a relatively heavy load can be lifted and, on the other hand, a relatively light load can be operated at a relatively high speed. This type of design means winches and motors are relatively light and cheap.
The object of the current invention is achieved by the fact that the hoisting cable is guided over loose pulleys that can be moved between a first position, in which the loose pulleys are connected to the mast, and a second position, in which the loose pulleys are connected to the trolley.
The effect of this measure is that the number of wire parts between the mast and the trolley can be set as desired. When the loose pulleys are attached to the mast, few wire parts will extend between the mast and the trolley, and a relatively low weight can be lifted with a relatively high speed. When the loose pulleys are attached to the trolley, a relatively large number of wire parts will extend between the mast and the trolley, and the trolley can be moved at a relatively low speed relative to the mast with a relatively large load. Since the hoisting cable is guided over the pulleys and the pulleys can be attached as desired to the mast or to the trolley, the hoisting cable does not have to be reeved again. The desired number of wire parts can be set in a relatively short time.
According to the invention, it is possible for the loose pulleys to be attached symmetrically relative to the center of the mast. This orientation of the attachments ensures that the forces exerted upon the cables are also transmitted symmetrically to a mast. The symmetry means that no additional bending loads are exerted upon the mast limiting the necessary weight of the mast.
According to the invention, it is also possible for the loose pulleys to be accommodated in a housing. Locking elements can be added for fixing the pulleys on the trolley. The loose pulleys are pulled automatically into their first position, in contact with the mast, by tension in the hoisting cable. It is therefore sufficient to provide the bottom side of the pulleys with locking elements. The use of a hydraulic actuation device means that the locking pins can be remotely controlled.
A relatively new technology is casing drilling. Casing drilling towers and drilling derricks need some special features such as openings in travelling blocks and crown blocks at the location of the firing line. In drilling derricks in the prior art, these special features can only be installed at high cost. An object of the present invention is to provide a hoist system of the type mentioned in the preamble by in which casing drilling operations can be performed without additional costs and with increased safety.